Static exciters are widely used for feeding the field winding of electric generators.
US2007/0296275 discloses, with reference to FIG. 1 of this document, a static exciter having an exciter transformer that is connected at one side to a busbar of a generator, i.e. to the output of the generator, and at the other side to a rectifier that converts the AC voltage fed from an exciter transformer into a direct current (DC) voltage.
The rectifier is typically driven by an Automatic Voltage Regulator (AVR) that is connected to the busbar via a voltage transformer.
The DC side of the rectifier is connected in series with the field winding of the generator and a forward biased diode.
The forward biased diode is connected in parallel with a capacitor bank. Moreover, a switch is provided between the capacitor bank and the diode, such switch can be closed to connect the capacitor bank in series with the rectifier.
During normal operation the switch is open, the alternating current (AC) voltage from the exciter transformer is converted into a DC voltage and is fed to the field winding because the diode is forward biased and conducts current.
In case a disturbance at the grid occurs, for example due to a short circuit in the transmission line, the voltage of the grid drops and, therefore, also the voltage of the busbar and the voltage at the lower voltage side of the voltage transformer drops. These disturbances are especially an issue with the recent rising of renewable energies feeding the electric grid and the corresponding higher requirements to grid safety.
When the voltage at the lower voltage side of the voltage transformer drops below a prefixed voltage value, the switch is closed such that the diode is reverse biased and does not conduct anymore, and the capacitor bank is connected in series with the rectifier and field winding.
This causes a considerably higher field voltage to be applied to the field winding and guarantees the stability of the generator/grid system.
Nevertheless, once a disturbance occurs, the capacitor bank is fully discharged, therefore if a plurality of disturbances occurs the system is not able to cope with.
In fact, even if the static exciter is provided with a charging device such as a battery that keeps the capacitor bank charged, the charging device dimension is usually small and it takes minutes to hours for it to recharge the capacitor bank to an acceptable charge level.
The document EP 2288017 B1 describes a static exciter of a field winding of an electric generator comprising a rectifier connected to an electric grid and to the field winding of the electric generator, an unidirectional electronic switch connected in series with the rectifier, and a capacitor bank. The static exciter further comprises a switch between the unidirectional electronic switch and the capacitor bank connected to a control unit that closes it when a voltage indicative of the grid voltage drops below a first prefixed voltage value to connect the capacitor bank to the field winding. The control unit opens said switch when a voltage indicative of the grid voltage exceeds a second prefixed voltage value to disconnect the capacitor bank from the field winding, such that the capacitor bank supplies energy to the field winding only for the time needed.
However, the solutions described in the state of the art are complex, require installation space, and are expensive.